A technique for multiplexing optical signals with a high density is studied so as to maximally utilize a transmission band of an optical fiber with an increase in demands for communication. As such a technique, for example, a Nyquist wavelength multiplexing system and a Coherent Optical-Orthogonal Frequency Divisional Multiplexer (CO-OFDM) system exist.
In the CO-OFDM system, interference between sub-carrier signals is suppressed by utilizing orthogonality between sub-carrier signals in which spectra thereof are adjacent to each other on a frequency axis. Thus, the CO-OFDM system, since sub-carrier signals of a high density may be multiplexed in a narrow frequency band, is expected to be a realizing unit of a theoretical limit (for example, 1 (bit/Hz) in a case of a binary code (intensity modulation system)) of a frequency utilization efficiency.
It is desirable to control a center frequency of each sub-carrier signal with high accuracy for generating a multiplexed optical signal (optical OFDM signal) multiplexed by the CO-OFDM system so that a spacing between the center frequencies of adjacent sub-carrier signals is equal to a symbol rate (baud rate) of the modulation.
Meanwhile, for example, a frequency control unit having a high accuracy that uses Single Side Band (SSB) modulation and Cross Phase Modulation (XPM) that is a nonlinear optical effect in an optical fiber is described in “Robert Elschner, Thomas Richter, Tomoyuki Kato, Shigeki Watanabe, Colja Schubert, “Distributed Coherent Optical OFDM Multiplexing Using Fiber Frequency Conversion and Free-Running Lasers”, OFC/NFOEC 2012, PDP5C.8, 2012”.